Inorganic oxide glass monoliths have found use in many applications such as optical filters, lasers, graded refractive index lenses, micro-optical arrays, waveguides, optical computers, non-linear optical elements, scintillation counters, polarizing filters, fiber optics, electro-optical components and other glass structures.
A recent development in the preparation of monolithic glasses and ceramics is the use of a "sol-gel" process to produce such articles. Because sol-gel monoliths are substantially homogenous in chemical composition and physical structure, porous glass monoliths produced by a sol-gel process have an advantageous combination of properties, and generally have superior optical properties as compared to porous glass monoliths prepared by other techniques. Further, sol-gel derived glass monoliths are substantially free of inorganic or organic impurities.
One of the process schedules for obtaining highly porous, consolidated silica sol-gel monoliths is shown schematically in FIG. 1. This process is more fully described in: (1) an article by L. Hench et al. entitled Gel-Silica Optics, 878 SPIE 76 (1988); (2) a paper by Larry L. Hench and Albert G. Fosmoe entitled Multifunctional Silica Optics, presented at the MRS fall meeting in Boston, Massachusetts on November 29, 1989; (3) an article by L. Hench et al. entitled The Sol-Gel Process, 90 Chem. Rev. 33 (1990); and (4) an article by J. Nogues et al. entitled Fast, Radiation-Hard Scintillatinq Detector: A Potential Application for Sol-Gel Glass, 17 J. Amer. Ceram. Soc. 1159 (1988). The disclosures of these documents are hereby expressly incorporated in their entirety by reference.
One of the disadvantages associated with the prior art is that cracking often occurs during the drying of an oxide gel. Although there is no generally accepted explanation for the phenomenon of cracking during drying, a variety of strategies have been devised to avoid such cracking, including: strengthening of the solid network by aging or by the addition of chemical additives; increasing permeability by increasing pore size; and reducing capillary pressure by either increasing pore size, reducing interfacial energies, or drying under supercritical conditions. Theory of Drying, Scherer, 73 J. Amer. Ceram. Soc. 3 (1990). Unfortunately, each of these methods has had limited success in practice, and each has some disadvantages, for example, in processing time or sintering temperature.
Thus, there remains a need in the art for a method of preparing oxide sol-gel monoliths which avoids the cracking problems of the prior art, yet still produces oxide sol-gel monoliths possessing all of the advantageous properties associated with this product.